As is known, pneumatic conveying systems are used, for example, for discharging powdered or granular material from the tank of a truck.
These conveying systems include at least one tube, through which pressurized conveying air flows, which extends between the discharge port of the tank and the end user of the powdered or granular product.
It is also known that in order to ease the emptying of the tank, aeration apparatuses are used preferably placed at the bottom of the tank itself.
The truck tank usually ends at the bottom with a discharge hopper which is often shaped as an upturned truncated cone. At the end of the truncated cone there is said discharge port of the powdered material with possibly a discharge valve.
Aeration apparatuses are usually used to ease the discharge of the material, arranged in the discharge hopper upstream of the discharge valve.
As will be better seen hereafter, each aeration apparatus is provided with a membrane made to vibrate by the output of compressed air in the annular gap between the inner surface of the tank wall and the membrane itself.
As is known, the vibration of membranes with the air flow coming out of the aeration apparatuses are used to break up the mass of particles present at the bottom of the tank and considerably accelerate the output of powdered material from the discharge port.
The above vibro-fluidization technology can normally be used successfully with food or chemical powders (starch, plastic, sugar, coffee, feed, sand, cement, aggregates, fine grit, etc.), all materials which tend to become compacted once stored inside containers.
However, in the solutions adopted so far by all the manufacturers, the outlet of micro-jets into the hopper takes place in all directions.
In other words, the compressed air micro-jets are directed downwards, sideways but also upwards, without having a preferential outlet direction. It was experimentally found that especially the micro-jets facing upwards, rather than easing and favoring the discharge of powdered material from the port of the hopper, somehow slow down the discharge as they are substantially faced in a direction opposite to the natural one of descent by gravity.
Quite recently, in order to make the action of the micro-jets more effective, aeration apparatuses of the above type have been proposed with vibrating membranes provided with substantially helical grooves arranged both on the outer surfaces of the membranes themselves, and on the inner ones. The aim of the inventors of this solution clearly was to create vortices within the granular (or powdered) mass so as to ease the discharge of the material through the discharge port.
However, in the manufacturing practice, it was noted that the inconsistent turbulence which is created in the mass of material partly obstruct the fall by gravity of the material to the discharge port. Moreover, it was experimentally verified that the output turbulent flows from the above annular gap cause an acceleration of the membrane deterioration due to the increased friction of the material (often highly abrasive, such as fine sand) on the inner and outer surfaces of the membrane itself. Moreover, other problems of different nature have been found in aeration apparatuses used in truck tanks.
In fact, in the solutions adopted so far, the aeration systems are fixed to the hopper wall by means of screw systems which provide the use of a threaded rod which causes a pulling action on the membrane as it is tightened by an operator. However, the force modulus with which the membrane is pressed on the inner surface of the hopper wall plays an important part in the whole process. In fact, if the tie rod subjects the membrane to an insufficient pull, there will be too much clearance between the membrane and the wall and therefore the membrane will not be efficiently made to vibrate by the entrance of the compressed air into the tank.
In use on trucks, it was found that the vibrations to which the aeration apparatuses are subjected during the movements of the truck itself cause a loosening of the pull on the membranes which eventually leads to a malfunctioning of the whole system.